1. Field of the Invention
The present invention relates to a helical antenna apparatus provided with two helical antenna elements, and to a radio communication apparatus provided with the same helical antenna apparatus. In particular, the present invention relates to a helical antenna apparatus for use in a mobile radio system, such as, mainly in a portable telephone, a radio transceiver for business use or the like, and a radio communication apparatus provided with the same antenna apparatus.
2. Description of the Prior Art
FIG. 22 is a perspective view showing one example of a situation in which a prior art portable radio transceiver 101 for business use is used. The VHF band of 150 MHz to 450 MHz is assigned as a radio frequency to the portable radio transceiver 101 for business use. Therefore, a normal-mode helical antenna apparatus 102 attached to the portable radio transceiver 101 is often employed as an antenna as shown in FIG. 22.
FIG. 23 is a circuit diagram showing an equivalent circuit of the helical antenna apparatus 102 for use in the portable radio transceiver 101 for business use of FIG. 22, and FIG. 23 includes an image of the helical antenna apparatus 102 of FIG. 22 inside of a radio transceiver housing.
Referring to FIG. 23, a helical antenna element 1 and a helical antenna element 2 are constituted so as to be symmetrical with respect to a feeding point, and have the same size parameters (winding diameter, number of turns, winding pitch) as those of each other. In this case, a capacitance element 3a having a predetermined fixed electrostatic capacity is connected between the helical antenna element 1 and the helical antenna element 2. By the capacitance element 3a and a balanced to unbalanced transformer 6, impedance matching is achieved between an input impedance Za of the helical antenna apparatus 102 and a coaxial cable 7 of a transmission line, and an impedance of the helical antenna apparatus 102 seen from an input connector 8 is set so as to become 50xcexa9 (See, for example, a prior art document of xe2x80x9cKoichi Ogawa et al., xe2x80x9cAn Analysis of the Effective Radiation Efficiency of the Normal Mode Helical Antenna Close to the Human Abdomen at 150 MHz and Consideration of Efficiency Improvementxe2x80x9d, The Transactions of the Institute of Electronics, Information and Communication Engineers in Japan, (B), Vol. J84-B, No.5, pp.902-911, May, 2001).
FIG. 24 is a graph showing a frequency characteristic of a voltage standing wave ratio (VSWR) in the helical antenna apparatus 102 of FIG. 23, and FIG. 24 illustrates the impedance characteristic of the helical antenna apparatus 102 designed for the 150 MHz band portable radio transceiver for business use. In this graph, the helical antenna elements 1 and 2 have a length of about 10 cm, and have an average shape as a portable radio transceiver on the market. As shown in FIG. 24, there is achieved an extremely good impedance matching state in which the VSWR is almost one at 150 MHz. However, the bandwidth in which the VSWR is equal to or smaller than two is within a range of 2 MHz, and this represents an extremely narrow band characteristic.
In general, the frequency assigned to the portable radio transceiver for business use has a range of 10 MHz and higher. Therefore, according to the impedance characteristic shown in FIG. 24, there arise such a problem that the actual gain of the helical antenna apparatus 102 is significantly reduced due to an impedance mismatching loss when the antenna apparatus is used at a frequency other than the frequency at which matching is achieved. In order to cope with this problem, the current measures are to prepare a plurality of helical antenna elements that have different center frequencies and obtain satisfactory impedance with respect to all the frequencies by replacing the antenna according to the operation frequency. As described above, the first problem of the helical antenna for business radio use is that that the impedance characteristic has a narrow range.
The feature in use of the portable radio transceiver for business use is that the radio transceiver is mounted on a human body so as not to hinder the business in a manner different from that of the portable telephone and the like. Upon having a telephone conversation using the radio transceiver, the user utilizes a microphone and an earphone as shown in FIG. 22. At this time, as is apparent from FIG. 22, the helical antenna apparatus 102 is brought into contact with the abdomen of the user 103. The antenna characteristics in this situation are described in detail in, for example, the above-mentioned prior art document, which was written by the present inventor and the others. The outline thereof will be described below.
FIG. 25A is a perspective view showing a positional relation between the helical antenna apparatus 102 and a human body model 201 of FIG. 23, and FIG. 25B is a Smith chart showing a range dependence characteristic of the input impedance Za of the helical antenna apparatus 102 of FIG. 23.
As shown in FIG. 25A, the helical antenna apparatus 102 is located so as to be close to the human body model 201 of an elliptic columnar configuration but be separated at a distance D. FIG. 25B shows calculated values of the input impedance Za when the distance D between the helical antenna apparatus 102 and the human body is changed, and the frequency is 150 MHz. As shown in FIG. 25B, the input impedance Za has its inductive reactance increasing as the helical antenna apparatus 102 approaches the human body. This is attributed to that the mutual inductance has equivalently increased as the results of an electromagnetic interaction between the helical antenna apparatus 102 and the human body.
FIG. 26 is a graph showing a loss power ratio with respect to the distance D between the human body and the antenna of the helical antenna apparatus 102 of FIG. 23, and FIG. 26 shows calculation results of various power losses of the helical antenna apparatus 102 appearing as the result of the impedance change shown in FIGS. 25A and 25B.
Referring to FIG. 26, Pt represents the summation of power losses, Pm represents a power loss due to impedance mismatching, Pa represents a power loss due to the metal resistance of the antenna, and Ph represents a power loss due to the electromagnetic absorption of the human body. The horizontal axis of FIG. 26 represents the distance D between the antenna and the human body, and the vertical axis represents the rate of each power loss (loss power ratio) with respect to the summation Pt of the power losses.
As is apparent from FIG. 26, if the helical antenna apparatus 102 approaches the human body, then the impedance mismatching loss Pm comes to share the greater part of the whole loss power in comparison with the metal conductor loss Pa of the antenna and the absorption power loss Ph of the human body. This is caused due to that the input impedance Za of the helical antenna apparatus 102 becomes remarkably large inductive as the distance D decreases, as shown in FIG. 25B. As the result of FIG. 26, the prior art document analytically describes that the radiation efficiency at a distance of D=2 cm has an extremely low value of equal to or smaller than xe2x88x9220 dB.
As is comprehensible from the above-mentioned analytical results, the other problem of the helical antenna apparatus 102 of FIG. 22 is an increase in power loss due to impedance mismatching in a situation in which a human body is located so as to be close to the apparatus.
As described above, the helical antenna apparatus 102 for business radio use has the following two problems. The first problem is the narrow range of the impedance characteristic, and the second problem is the increase in power loss due to impedance mismatching when a human body is located so as to be close to the apparatus. These two problems are each attributed to the impedance mismatching between the input impedances Za of the helical antenna apparatus 102 and the impedance of the transmission line connected to the helical antenna apparatus 102.
However, in the helical antenna apparatus 102 of the prior art example shown in FIG. 23, the impedance matching has been achieved only at the specified frequency predetermined in free space, and this has therefore led to such a problem that the impedance frequency characteristic has had a narrow range. Furthermore, there has been such a problem that, in the situation in which the helical antenna apparatus 102 has been located so as to be close to a human body, the mismatching situation has been promoted by the electromagnetic interaction between the helical antenna apparatus 102 and the human body even at the frequency at which the impedance matching is achieved in free space and the actual gain of the antenna has been significantly reduced.
An essential object of the present invention is to solve the above-mentioned problems and provide a helical antenna apparatus, capable of being used in a wide band and of reducing the power loss due to impedance mismatching when the antenna is located so as to be close to a human body, and a radio communication apparatus provided with the same helical antenna apparatus.
In order to achieve the above-mentioned objective, according to one aspect of the present invention, there is provided a helical antenna apparatus connected to either one of a balanced feeder line and a balanced port of a balanced to unbalanced transformer of a feeder circuit. The helical antenna apparatus includes a first helical antenna element, a second helical antenna element, first to third variable capacitance elements. The first variable capacitance element is connected between the first helical antenna element and the second helical antenna element, and the second variable capacitance element is connected between (a) either one of the balanced feeder line and a first terminal of the balanced port of the balanced to unbalanced transformer, and (b) the first helical antenna element. The third variable capacitance element is connected between (a) either one of the balanced feeder line and a second terminal of the balanced port of the balanced to unbalanced transformer, and (b) the second helical antenna element.
The above-mentioned helical antenna preferably further includes a detector and an adaptive controller. The detector is connected between (a) either one of the balanced feeder line and the feeding port of the balanced to unbalanced transformer, and (b) a radio transmitter. The detector detects at least one detection value of a reflection signal reflected from the first and second helical antenna elements when the first and second helical antenna elements are fed with a transmission signal from the radio transmitter, a reflection coefficient and a voltage standing wave ratio. The adaptive controller adaptively controls respective capacitance values of the first, second and third variable capacitance elements, so that either one of the detected detection value and a predetermined estimation function including the reflection signal becomes substantially minimized.
According to another aspect of the present invention, there is provided a helical antenna apparatus connected to an unbalanced feeder line, and provided on a radio communication apparatus housing. The helical antenna apparatus includes a helical antenna element, and first and second variable capacitance elements. The first variable capacitance element is connected between the helical antenna element and the radio communication apparatus housing, and the second variable capacitance element connected between the unbalanced feeder line and the helical antenna element.
The above-mentioned helical antenna apparatus preferably further includes a detector and an adaptive controller. The detector is connected between the unbalanced feeder line and a radio transmitter, and the detector detects at least one detection value of a reflection signal reflected from the helical antenna element when the helical antenna element is fed with a transmission signal from the radio transmitter, a reflection coefficient and a voltage standing wave ratio. The adaptive controller adaptively controls respective capacitance values of the first and second variable capacitance elements, so that either one of the detected detection value and a predetermined estimation function including the reflection signal becomes substantially minimized.
According to a further aspect of the present invention, there is provided a radio communication apparatus, which includes a helical antenna apparatus, a radio transmitter, a radio receiver. The helical antenna apparatus is connected to either one of a balanced feeder line and a balanced port of a balanced to unbalanced transformer of a feeder circuit. The radio transmitter is connected to the helical antenna apparatus, and the radio receiver connected to the helical antenna apparatus. The helical antenna apparatus includes first and second antenna elements and first to third variable capacitance elements. The first variable capacitance element is connected between the first helical antenna element and the second helical antenna element. The second variable capacitance element is connected between (a) either one of the balanced feeder line and a first terminal of the balanced port of the balanced to unbalanced transformer, and (b) the first helical antenna element. The third variable capacitance element is connected between (a) either one of the balanced feeder line and a second terminal of the balanced port of the balanced to unbalanced transformer, and (b) the second helical antenna element.
In the above-mentioned radio communication apparatus, the helical antenna apparatus further includes a detector and an adaptive controller. The detector is connected between (a) either one of the balanced feeder line and the feeding port of the balanced to unbalanced transformer, and (b) a radio transmitter, and the detector detects at least one detection value of a reflection signal reflected from the first and second helical antenna elements when the first and second helical antenna elements are fed with a transmission signal from the radio transmitter, a reflection coefficient and a voltage standing wave ratio. The adaptive controller adaptively controls respective capacitance values of the first, second and third variable capacitance elements, so that either one of the detected detection value and a predetermined estimation function including the reflection signal becomes substantially minimized.
The above-mentioned radio communication apparatus further includes a controller apparatus, which controls operation of the radio transmitter and the radio receiver, wherein the controller apparatus includes the adaptive controller.
According to a still further aspect of the present invention, there is provided a radio communication apparatus which includes a helical antenna apparatus connected to an unbalanced feeder line and provided on a radio communication apparatus housing, a radio transmitter connected to the helical antenna apparatus and a radio receiver connected to the helical antenna apparatus. The helical antenna apparatus includes a helical antenna element, and first and second variable capacitance elements. The first variable capacitance element is connected between the helical antenna element and the radio communication apparatus housing, and the second variable capacitance element connected between the unbalanced feeder line and the helical antenna element.
In the radio communication apparatus, the helical antenna apparatus preferably further includes a detector and an adaptive controller. The detector is connected between the unbalanced feeder line and a radio transmitter, and the detector detects at least one detection value of a reflection signal reflected from the helical antenna element when the helical antenna element is fed with a transmission signal from the radio transmitter, a reflection coefficient and a voltage standing wave ratio. The adaptive controller adaptively controls respective capacitance values of the first and second variable capacitance elements, so that either one of the detected detection value and a predetermined estimation function including the reflection signal becomes substantially minimized.
The above-mentioned radio communication apparatus preferably further includes a controller apparatus, which controls operation of the radio transmitter and the radio receiver, wherein the controller apparatus includes the adaptive controller.